(feat): add optional database indexes

[bgervan]

🔗 Linked issue

❓ Type of change

• [x] 📖 Documentation (updates to the documentation or readme)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Adds option to generate indexes on the generated SQL schemas, making it possible to optimize D1 row counts for example.

📝 Checklist

• [ ] I have linked an issue or discussion.
• [x] I have updated the documentation accordingly.
• [x] I have added test cases

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npm i https://pkg.pr.new/@nuxt/content@3637

commit: 01ad7a0